The project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in motor control, as studied in adult cats in vivo and in the isolated brain stem and spinal cord of neonatal rats or mice studied in vitro. Current interest centers on the organization of excitatory last-order interneurons in reflex pathways within the spinal segment and control of information flow in them by input from primary afferent and supraspinal descending systems, and their interaction with the spinal mechanisms that generate rhythmic motoneuron output patterns underlying locomotion. In the cat, we are particularly concerned with interneurons that transmit short-latency excitation from low-threshold skin afferents and from reticulospinal and vestibulospinal systems that all produce minimally disynaptic excitation in some species of lumbosacral motoneurons. All these interneuron groups are strongly influenced by the spinal central pattern generator (CPG) for locomotion. The differential patterns of CPG modulation indicate that separate systems of segmental interneurons, each with highly specific patterns of primary afferent and descending convergence, are present in the mammalian spinal cord. Work on the in vitro preparation of neonatal rodent spinal cord, with or without the brain stem, is in its early stage and is aimed at developing preparations that can be used to investigate functionally defined reflex pathways, in order to compliment work in the cat. We hope to develop techniques suitable to study neural systems activated by particular peripheral nerves and descending systems, with extracellular recording from identified peripheral nerves and intracellular recording from select populations of motoneurons and interneurons. Early results have been encouraging and promise to provide a means to study specific neural systems under conditions that are impossible to achieve in the in vivo situation.